Solid high energy fuel compositions containing boranes and rubber



3,122,461 SOLID HIGH ENERGY FUEL CGMPQSITEUNS CONTAINING BORANIES AND RUBBER Eugene J. De Lorenzo and William L. Waelitel, Niagara Falls, N .Y., assignors to Ulin Mathieson Chemical 60rporation, a corporation of Virginia No Drawing. Filed July 18, E58, Ser. No. 749,564

14 tfllairns. (Cl. 14922) This invention relates to reaction products of natural rubber with lower alkyl decaboranes or with lower alkyl decaboranes and decaborane.

It has now been found that natural rubbers can be combined with lower alkyl decaboranes or with lower alkyl decaboranes and decaborane to form a solid material of high boron content suitable for use as a propellant fuel for rocket power plants and other jet propelled devices.

The fuels of this invention, when incorporated with suitable oxidizers such as ammonium perchlorate, potassium perchlorate, sodium perchlorate, ammonium nitrate, etc., burn with high flame speeds have high heats of combustion and are of the high specific impulse type. Probably the single most important factor in determining the performance of a propellant charge is the specific impulse. Appreciable increases in performance will result from the use of higher specific impulse materials. The fuels of this invention when incorporated with oxidizers are capable of being formed into a wide variety of tablets, rings and shapes with all desirable mechanical and chemical properties. Propellants produced by the methods described in this application, burn uniformly without disintegration when ignited by conventional means, such as a pyrotechnic type igniter, and are mechanically strong enough to withstand ordinary handling.

An important advantage of the fuels of this invention is that they are partially elastic and hence resist cracking by heat and thermal shock. In practical use, solid propellants for jet propelled devices may be exposed to a variety of temperature conditions. Wide variations in temperatures cause thermal expansions which may induce cracking in the propellant grains. Similarly, physical shock experienced by the propellant grains both before and during their discharge in jet propelled devices may cause fracture and cracking of the grains. Such cracked propellant grains can be extremely dangerous to the jet propelled device in that they permit the grain to burn more rapidly in the immediate vicinity of the cracked section. The uneven burning rate which results may be so great as to cause an uneven thrust by the jet engine or an explosion in the rocket power plant. It is therefore very essential that the propellant used not be subject to fracture. Compounds having a rubber-like quality are particularly desirable for use in propellant grains because they can absorb both thermal and mechanical shock without cracking or fracturing. The natural resiliency and elastic qualities of rubber-like materials make them ideal for resisting fracture. The fuel of this invention will not deteriorate when stored over a wide range of temperature conditions. It can be produced in a uniform composition, can easily be cast into grains and will not shrink. In addition, the high percentage of boron which is present-in the propellant composition is in a readily burnable form and results in the release of much larger quantities of energy than the corresponding composition of carbon. The fuels of this invention need not use the conventional binding agent used by many other similar compounds to cause the oxidizer and fuel to adhere. The cohesive and tacky quality of the final product is sufiicient to bind into a solid mass the granular oxidizing agent which is admixed States Patent ice with the fuel. Such mass will retain its form and elastic qualities when shaped into various configurations.

According to this invention, a minor amount of a natural rubber is reacted with a major amount of a lower alkyl decaborane and cured to produce solid materials useful as propellant fuels. Also, a solid natural rubber can be reacted with a lower alkyl decaborane to form a plasticized product which is then reacted with decaborane and cured. In this reaction the total amount of alkyl decaborane and decaborane used is a major amount and the amount of rubber is a minor amount. This solid product is advantageous as a fuel in that more boron can be incorporated into the resulting solid.

The natural rubber is preferably used in the form of crepe to facilitate subdividing before reaction with the alkyl decaborane, although other forms can be used. The useful natural rubbers have an average molecular weight range of about 25,000 to about 1,500,000.

The lower alkyl decaboranes suitable for use in this invention are those in which the alkyl group contains from 1 to 6 carbon atoms. Such alkyl decaboranes include, for example, monomethyldecaborane, dimethyldecaborane, monoethyldecaborane, diethyldecaborane, triethyldecaborane and monoisopropyl decaborane. Mixtures of lower alkyl decaboranes can be used. Alkyl decaboranes can be prepared by reacting decaborane and an alkyl halide in the presence of an alkylation catalyst as described in pending application Serial No. 497,407, filed March 28, 1955, of Altwicker et al., and in pending application Serial No. 540,141, filed October 12, 1955, of Altwicker et al., wherein ferric chloride is used as the the catalyst. Monoethyldecaborane can be also prepared by the reaction of decaborane and ethylene as described in pending application Serial No. 514,121, filed June 8, 1955, of Stange et al. Also alkylated decaboranes such as monoethyldecaborane and diethyldecaborane can be prepared by reacting a mono olefin hydrocarbon with a mixture of decaborane and an aluminum halide as described in pending application Serial No. 557,634, filed January 6-, 1956, of Neif et al.

In the reaction of the rubber with alkyl decaborane, or alkyl decaborane and decaborane, both reaction and plasticizing take place. The product before curing is a soft pliable substance. This substance can be readily cured by conventional means to provide a non-tacky, strong, boron-containing solid material having elastic and rubber-like qualities. The boron-containing solid material can be formed either by first producing the uncured reaction product, i.e. by admixing the rubber and alkyl decaborane and subjecting the mixture to suitable temperatures and pressures, and then curing it or by carrying out the reaction of the rubber and alkyl decaborane under curing conditions of temperature and pressure with or without the use of a curing agent. Also, as noted above the plasticized reaction product of the rubber and lower alkyl decaborane can be further reacted with decaborane.

The curing agents that can be used include, for example, the alkyl decaborane itself, the decaborane, and the conventional sulfur cures or any other cures applicable to natural rubber. If the alkyl decaborane or decaborane is used as the curing agent the reaction is preferably carried out under curing conditions of temperature and pressure, although cured material can be obtained by allowing the reaction product to stand for a suitable period of time. If extraneous materials are used as curing agents they can be either admixed with the rubber and alkyl decaborane and the reaction carried out under curing conditions of temperature and pressure to provide directly the boron-containing cured solid material or the agents can be admixed with the separately formed reaction product and the mixture subjected to curing conditions.

In the preparation of the compositions containing an oxidizer, the oxidizer can be added to the reaction mixture, including the curing agent if it is present, and the reaction and curing carried out in the presence of the oxidizer or the oxidizer can be added to the separately formed'reaction product before curing or the oxidizer can be added to the separately formed and cured product, e.g. by mixing it into the product.

The rubber is generally reacted with the alkyl decaborane at temperatures ranging from about 20 to about 200 C. and at atmospheric pressure, although elevated pressures, e.g. from 3 to 1,500 atmospheres, can be used particularly when curing is eifected as Well as reaction. The time of reaction varies from about 1 to 36 hours or more, depending on the temperature and pressure employed. A major amount of the alkyl deca'borane or alkyl decaborane and decaborane is used. The amount of alkyl decaborane, when it is the sole boron reactant, can vary from more than 1 to about 6 parts, preferably from about 1.5 to about 3.5 parts, by weight per part of rubber. The amount of decaborane used, when decaborane is reacted with the alkyl decaborane-plasticized rubber, can vary from about 0.001 to about 4 parts, preferably about 0.01 to about 1 part, by weight per part of rubber, the total amount of alkyl decaborane and decaborane being a major amount, i.e. more than 1 part per part of rubber. The amount of alkyl decaborane used for plasticizing the rubber for use in the reaction with decaborane can vary from about 0.001 to about 6 parts, preferably about 0.5 to about 3.5 parts, by weight per part of rubber. The reaction of the plasticized product (the reaction product of rubber with alkyl decaborane) with decaborane is carried out under conditions similar to those of reaction of the rubber with alkyl decaborane.

The reaction can be also conducted in the presence of inert solvents such as me-thylcyclohexane. Such solvents can be introduced into the reaction vessel with the reactants or the borane or rubber can be dissolved in the solvents and the solution introduced into the reaction vessel with the borane or rubber. Generally, the solvent is used in as amount of about 50 to 500 weight percent of solvent based on the reactants.

When a sulfur cure is utilized, temperatures of about 20 to 200 C. and pressures from about 50 p.s.i.g. to about 20,000 p.s.i.g. can be used. When the alkyl decaborane or decaborane is used as the curing agent, temperatures of about 20 to 200 C. and pressures from about 50 p.s.i.g. to about 9,000 p.s.i.g. can be used. The time of curing utilizing the sulfur or borane cure varies generally from about 0.25 to about 36 hours.

The solid materials produced by the reaction described above are elastic or rubber-like solids of varying hardness and colors. Some aretacky. They are non-pyrophoric but burn readily on ignition. The solids contain a substantial amount of boron and some of the boron is linked to the rubber molecule, probably between two or more rubber molecules.

This invention will be further illustrated by the following examples.

Example I 11.8 grams of natural rubber (pale crepe No. 1- Hevea) was milled for one hour in a mechanical rubber mill, then placed in a 500 ml. glass flask and 150 ml. of methylcyclohexane added. The flask was placed on a mechanical agitator for 24 hours to insure the complete dissolution of the rubber. 6.8 ml. of the solution (whose total volume was 163 ml.) were then blended with 1.1 grams of monoethyldecaborane and placed in a glass test tube shaped reactor about inches long. The reactor was placed in an oil bath and maintained at a temperature of about 133 for five hours. The solution thickened gradually leaving a gel at the conclusion of the experiment. After cooling, the methylcyclohexane was extracted from the solution by exposing to a partial vacuum (1-2 mm. of mercury) for seven hours. The remaining solid was a pale yellow elastomer, which did not exude fluid when squeezed. When ignited, it burned vigorously with an orange-green flame.

Example II A mixture of 5.0 grams of natural rubber (polyisoprene) and 7.0 of monoethyldecaborane is made using a mortar and pestle for mixing. To a 12 g. portion of this mixture 0.5 g. of decaborane is added. This admixture is accomplished on a hand mill consisting of two steel rolls 1" x 3" which are at constant speeds. The mixture is cured or heated at 55 C. in a Carver press for 16 hours. A firm molded slab is produced. Gel content and boron content on the gel is obtained. Gel content is determined by equilibration of the sample in benzene at room temperature. Gel content is about percent and boron in the gel is about 5 percent or 8 percent calculated as ethyldecaborane.

The rubber used in Example II had the following properties:

Formula of repeating unitCH -[CHOH=CCH]u Density at 25 C., g./ cc 0.91 Refracitve index at 25 C 1.5191 Average molecular wt 400,000 Dilute solution viscosity 2.06

The alkylated decaboranes used in Examples I and II were obtained from a Friedel-Crafts reaction of decaborane-14 (B H and an ethylhalide, in the presence of a ferric chloride catalyst. The product of this reaction consisted of a mixture of ethyl decaborane, diethyldecaborane and triethyldecabo rane. These constituents were separated by a fractional distillation at reduced pressures. At a pressure of about 0.2 mm. of mercury, monoethyldecaborane boils at approximately 30 C., diethyldecaborane at 45 C. and triethyldecaborane at 65 C. By this method of separation products were obtained having a purity of greater than 99 percent.

The boron containing solid materials of this invention can be employed as ingredients of solid propellant compositions in accordance with general procedures which are Well understood in the art, since the solids are readily oxidized using conventional solid oxidizers, such as ammonium perchlorate, potassium perchlorate, sodium perchlorate, aluminum perchlorate, lithium perchlorate, ammonium nitrate and the like. In formulating a solid propellant composition employing one of the solid materials generally from 10 to 35 parts by weight of the boron containing material and from 65 to parts by weight of oxidizer such as ammonium perchlorate are present in the final propellant composition.

In the propellant, the oxidizer and the product of this invention are formulated by intimately admixing the oxidizer with the reactants, i.e. the rubber and alkyl decaborane, before reaction and curing so that the oxidizer and boron-containing product form a homogeneous mass or by intimately admixing the oxidizer with the uncured or cured reaction product which has been prepared without oxidizer, e.g. by finely subdividing each of the materials separately and thereafter intimately admixing them. The purpose in doing this, as the art is aware, is to provide a proper burning characteristic in the final propellant. The subdividing of the boron-containing solid reaction product can be accomplished by means of conventional equipment designed to reduce the size of resilient materials, e.g. rubber, such as devices which cut, chop or tear the feed material into a divided product, e.g. the well-known rotary knife cutters and granulators. The oxidizer can be reduced in size by conventional equipment, e.g. a hammer mill. The admixing of the finely divided reaction product and finely divided oxiizer can be accomplished by conventional mixing equip- 5 ment employing rotating blades e.g. kneaders or Baker- Perkins mixers.

In addition to the oxidizer and oxidizable material, the final propellant can contain an artificial binder. Although, in general, it is not necessary to use such a binder when the fuel of this invention is utilized, one can be employed to secure additional binding strength, or where a propellant fuel of different physical characteristics is desired. A binder of an artificial resin type, generally containing urea-formaldehyde or phenol-formaldehyde, is particularly useful. The function of this resin is to give the propellant mechanical strength and at the same time improve its burning characteristics. In manufacturing a suitable propellant using a binder, the cured re action product containing the oxidizer or proper proportions of finely divided oxidizer and finely divided boroncontaining material containing no oxidizer are admixed with a high solids content solution of a partially condensed urea-formaldehyde or phenol-formaldehyde resin, the proportion being such that the amount of the resin is about from 5 to percent by weight, based upon the weight of the oxidizer and the boron compound. The ingredients are thoroughly mixed with the simultaneous removal of the solvent, e.g. by use of a vacuum, and following this the solvent-free mixture is molded into the desired shape as by extrusion. Thereafter, the resin in the mixture can be cured, e.g. by heating at moderate temperatures, e.g. 100200 C. Conventional and suitable methods of formulation of solvent propellant compositions are further described in US. Patent 2,622,277, of Bonnell, and U.S. Patent 2,646,596, of Thomas.

When the final propellant is to be made without binder or binding agent, the technique is very similar. When the boron-containing product has been made without oxidizer the oxidizer and product are again formulated in intimate admixture with each other, as by subdiving each of the materials separately and then intimately admixing them. The result of this admixture is a homogeneous solid having strong cohesive qualities, which will retain its form well, and which can be formed into various shapes as by extrusion. The molding can be done in conventional equipment designed for this purpose such as hydraulic extruders. The extruded propellant can then be cut into pieces and trimmed as desired.

The following examples illustrate the formulation of solid propellant compositions utilizing the solid boron containing reaction product of this invention which has been prepared without oxidizer.

Example III The solid boron containing material of Example I in an amount of 2.0 pounds is finely divided to a size of about 50 microns in a rotary knife cutter. Ammonium perchlorate (6.0 pounds) oxidizer is finely divided to a size of about 1 to 50 microns in a hammer mill. The two finely divided materials are then introduced into a Baker-Perkins mixer and intimately admixed. The resulting homogeneous cohesive solid is molded and extruded and cut into cylindrical grains of about 2 inches in diameter and 8 inches in length. The resulting solid grains are an intimate admixture of solid fuel and oxidizer.

Example IV The solid boron containing material of Example I and ammonium perchlorate oxidizer in the amounts of Example III are finely divided as in Example III and are introduced into a Baker-Perkins mixer along with 1.0 pound of a solution of a pheno ormaldehyde resin in epichlorohydrin and the three components intimately admixed with the simultaneous removal of the solvent by means of a vacuum. The resulting solvent-free homogeneous solid is molded and extruded to form grains as in Example III. The grains are then heated in an oven to about 100 to 150 C. for one hour to cure the resin in the grains. The resulting grains are an intimate admixture of solid. fuel and oxidizer with resin binder.

What is claimed is: t

1. A solid high energy fuel composition consisting essentially of the product of reaction of natural rubber and a lower alkyl decaborane at a temperature of about 20 to about 200 C. and in amounts of more than 1 to about 6 parts by weight of lower alkyl decaborane per part of rubber.

2. The composition of claim 1 in which the rubber and lower alkyl decaborane are reacted in amounts of about 1.5 to about 3.5 parts by weight of lower alkyl decaborane per part of rubber.

3. The composition of claim 2 in which the lower alkyl decaborane is monoethyldecaborane.

4. A solid high energy fuel composition consisting essentially of the product of reaction of natural rubber with a lower alkyl decaborane and decaborane at a temperature of about 20 to about 200 C., the total amount of lower alkyl decaborane and decaborane being more than 1 part by weight per part of rubber.

5. The composition of claim 4 in which the rubber, lower 'alkyl decaborane and decaborane are reacted in amounts of about 0.001 to about 6 parts by weight of lower alkyl decaborane per part of rubber and about .001 to about 4 par-ts by Weight of decaborane per part of rubber.

6. The composition of claim 4 in which the rubber, lower alkyl decaborane and decaborane are reacted in amounts of about 0.5 to about 3.5 parts by weight of lower alkyl decaborane per part of rubber and about 0.01 to about 1 part by weight of decaborane per part of rubber.

7. The composition of claim 6 in which the lower alkyl decaborane is monoethyldecaborane.

8. The method of preparing a solid high energy fuel composition which comprises reacting natural rubber with a lower alkyl decaborane at a temperature of about 20 to about 200 C. and in amounts of more than 1 to about 6 parts by weight of lower alkyl decaborane per part of rubber.

9. The method of claim 8 in which the rubber and lower alkyl decaborane are reacted in amounts of about 1.5 to about 3.5 parts by weight of lower alkyl decaborane per part of rubber.

10. The method of claim 9 in which the lower alkyl decaborane is monoethyldecaborane.

11. The method of preparing a solid high energy fuel composition which comprises reacting natural rubber with a lower alkyl decaborane followed by reaction with decaborane, said reactions being carried out at a temperature of about 20 to about 200 C. and the total amount of lower alkyl decaborane and decaborane being more than 1 part by weight per part of rubber.

12. The method of claim 11 in which the rubber, lower alkyl decaborane and decaborane are reacted in amounts of about 0.001 to about 6 parts by weight of lower alkyl decaborane per part of rubber and about 0.001 to about 4 parts by weight of decaborane per part of rubber.

13. The method of claim 11 in which the rubber, lower alkyl decaborane and decaborane are reacted in amounts of about 0.5 to about 3.5 parts by weight of lower alkyl decaborane per part of rubber and about 0.01 to about 1 part by weight of decaborane per part of rubber.

14. The method of claim 13 in which the lower alkyl decaborane is monoethyldecabo-rane.

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1. A SOLID HIGH ENERGY FUEL COMPOSITION CONSISTING ESSENTIALLY OF THE PRODUCT OF REACTION OF NATURAL RUBBER AND A LOWER ALKYL DECABORANE AT A TEMPERATURE OF ABOUT 20 TO ABOUT 200*C. AND IN AMOUNTS OF MORE THAN 1 TO ABOUT 6 PARTS BY WEIGHT OF LOWER ALKYL DECARBORANE PER PART BY RUBBER. 